The invention is directed to a process for the production of oxymethylene copolymers-molding compositions with a low content of residual formaldehyde by heating the polymerizate melt treated with chemical stabilizers in an apparatus in which new polymerizate surfaces are continuously presented under reduced pressure.
Oxymethylene copolymerizates are produced on a large commercial scale by cationic copolymerization of trioxane with suitable comonomers. The crude copolymerizate still contains unreacted monomers, residues of the catalyst used and thermally unstable portions. They therefore still need a special working up in order to be used and processed as thermoplastic molding compositions in customary manner.
It is already known to carry out the working up, particularly the removal of the thermally unstable portion in such manner that the polymerizate melt treated with chemical stabilizers is heated under reduced pressure in an apparatus in which there are continuously presented new polymerizate-surfaces (German AS No. 1,445,273, (the entire disclosure of which is hereby incorporated by reference and relied upon). As suitable apparatus there are mentioned roller mixing mills and extruders. In the course of time the known processes on the other hand were still further improved by carrying out the processing in special degassing extruders. The molding compositions worked up according to the known processes contain a relatively high portion of residual formaldehyde which frequently is the cause of odor problems in the further processing of deposits on the walls of the injection molds and in producing molded articles with faults.
Therefore, there was also already known a process by which the odor forming residual formaldehyde is removed from the polyoxymethylene-molding composition in granulate form in a special subsequent treating step (Doerfel, German AS No. 2,435,384). This additional treatment, however, is quite expensive and particularly requires a relatively long treatment time.